1. Field of the Invention
The present invention relates to a fiber Bragg grating (FBG) sensor system configured by combining a wavelength-tunable light source capable of fast wavelength sweep and a photodetector with an FBG provided to a measurement object at a remote site via a fiber, thereby to perform, for example, distortion measurement and temperature measurement of a measurement object of interest. More specifically, the invention relates to an FBG sensor system capable of performing accurate measurement of the measurement object even when the length of fiber to the FBG exceeds 80 km in the manner that the influence of Rayleigh scattered light occurring in a fiber is reduced by using optical pulses for measurement light incident on the FBG.
2. Description of the Related Art
An FBG is formed by varying a refraction index of a core portion of a predetermined length of a fiber periodically at a fixed interval. When incident light is received at one-end portion of the FBG, only a specific wavelength (“Bragg wavelength”, hereinbelow) of the incident light is reflected, and light parts of other wavelengths are transmitted. The Bragg wavelength is variable depending upon or corresponding to an axial distortion (compression or expansion) introduced to the portion where the refraction index periodically varies at the fixed interval. Therefore, the distortion imposed on the FBG can be measured by measuring the wavelength (reflection wavelength) of light incident on the one-end portion of the FBG or the wavelength of transmissive light.
Conventionally known FBG sensor systems for performing distortion measurement, temperature measurement, and the like by using the properties of the FBG as described above include those of the type formed by combining a Littmann wavelength-tunable light source and a photodetector to measure the reflection wavelength of the FBG. In recent years, the applicant of the present invention has proposed an FBG sensor system that, in lieu of the Littmann wavelength-tunable light source, uses a MEMS (Microelectromechanical Systems) scanner using a wavelength-tunable light source enabling fast wavelength sweep, thereby to increase the measurement speed (see Japanese Patent Application Laid-Open (JP-A) No. 2006-49785). The MEMS scanner is a scanner formed from a microelectromechanical structure (structure mechanically operating under control of an electric signal).
FIG. 12 shows an overall configuration of the conventionally proposed FBG sensor system using the wavelength-tunable light source, shown by reference numeral 10, which enables the fast wavelength sweep. In the wavelength-tunable light source 10, light emitted from an AR-coated facet (AR: anti-reflection) of a semiconductor laser (LD) 1 is converted by a collimating lens 2 to collimated light, and is directed to be incident on a diffraction grating 3. Then, diffracted light of the incident light, which has been output from the diffractive grating 3, is incident on a MEMS scanner 60. The MEMS scanner 60 is configured from a reflector 35 and reflector driving means 50. The diffracted light of the collimated light incident from the diffractive grating 3 is reflected off of a reflective surface of the reflector 35 to the diffractive grating 3 and is diffracted again by the diffractive grating 3, and the diffracted light thus obtained is incident on the LD 1 through the collimating lens 2. In this case, the reflector driving means 50 works such that the angle of the reflective surface of the reflector 35 is reciprocally rotated in a predetermined sweep period so that the diffracted light incident on the LD 1 becomes light of a desired wavelength and the desired wavelength including a predetermined wavelength range is reciprocally swept.
According to the configuration described above, wavelength-swept light is oscillated and output from a non-AR-coated facet to function as output light (measurement light). The reflector driving means 50 itself generates a drive signal (determining the wavelength range and the sweep period) to reciprocally rotate the angle of the reflective surface of the reflector 35, and outputs the drive signal as a sweep signal “a” to a processing unit 17.
Zero order light from the diffractive grating 3 is incident on an optical resonator 4 of an etalon or the like, and only light of a predetermined wavelength is transmitted therethrough. The transmitted light is then converted by a photodetector (PD) 5 to an electric signal b, and is then output to the processing unit 17. More specifically, transmitted light at a predetermined wavelength interval of, for example, 15 GHz, is generated corresponding to the wavelength sweep of the output light (measurement light), and the light is photoelectrically converted by the photodetector 5 to the electric signal b. The wavelength (frequency) of the transmitted light is known. Accordingly, an oscillation wavelength (wavelength of the wavelength-swept measurement light) of the wavelength-tunable light source 10 can be obtained by use of the electric signal b, which has been obtained by the photoelectrical conversion, and the sweep signal a.
Subsequently, an optical circulator 13 directs the measurement light supplied from the wavelength-tunable light source 10 to be incident on an FBG 15 through a fiber 14. In addition, upon receipt of reflected light (reflection spectrum) reflected off of the FBG 15 and returned therefrom, the optical circulator 13 outputs the light to a photodetector (PD) 16. The photodetector 16 photoelectrically converts the reflected light to an electric signal c and outputs the electric signal c to the processing unit 17.
The processing unit 17 measures the reflection wavelength of the FBG 15 in accordance with the electric signal c received from the photodetector 16, the sweep signal “a” and electric signal b received from the wavelength-tunable light source 10.
An exemplary case is now assumed that the conventional FBG sensor system measures a reflection wavelength of a wavelength range (measurement wavelength range) of 1520 to 1580 nm for an FBG 15 having a reflection wavelength of 1550 nm. In this case, as shown in FIGS. 10A and 10B, light having a wavelength range (sweep wavelength range) of 1500 to 1600 nm inclusive of the measurement wavelength range is wavelength-swept to be a sine wave shape by the drive signal (sweep signal a) having a sweep period of 1.4 ms (714 Hz). Then, the light wavelength-swept to be the sine wave shape is directed to be continually incident on the fiber 14 as the measurement light.
A noise floor of the reflected light (reflection spectrum) of the measurement light returned by being reflected off of the FBG 15, is caused by the Rayleigh scattered light occurring in the fiber 14. The intensity of the Rayleigh scattered light is proportional to the intensity of the measurement light incident on the fiber 14. Accordingly, the signal-to-noise (S/N) ratio of the reflection spectrum of the FBG 15 is not dependant upon the intensity of the measurement light incident on the fiber 14.
Consequently, in the case where the measurement light shown in FIG. 10B is continuously input into the fiber 14, the S/N ratio of the reflection spectrum of the FBG 15 is represented by Equation (3) in accordance with a reflection spectrum intensity PF given by Equation (1) and Rayleigh scattered light intensity PR given by Equation (2) (provided that the reflectance of the FBG 15 is 100%):PF=P0e−2αL  (1)PR=RP0(1−e−2αL)/2  (2)S/N=PF/PR=2e−2αL/{R(1−e−2αL)}  (3)where, P0 denotes an intensity of the measurement light input into the fiber 14; L denotes a fiber length; α denotes a fiber attenuation factor; and R denotes a Rayleigh scattered light occurrence rate. In a case of a general optical communication fiber, α is 0.046/km (=0.2 dB/km) and R is 0.0022.
FIG. 11 shows, by way of a conventional example, calculated values of S/N ratios obtained from Equation (3) for the fiber length (L) and corresponding measurement values. As can be seen from FIG. 11, the S/N ratio is smaller as the fiber length is larger, and is 0 dB when the length is a range of about 60 km to 70 km. In practice, an S/N ratio in a range of about 10 dB to 20 dB is necessary to accomplish an accurate measurement of variations of the reflection wavelength (wavelength of the reflection spectrum) of the FBG 15. As a consequence, as seen from FIG. 11, a problem arises in that the measurable fiber length limit is restricted to about 30 km.